This invention relates to the preparation of rods of uniform density from powders of refractory materials, and, more particularly, to the preparation of such rods for use as preforms for growing single crystals of the refractory material.
Single crystals are solids having only a single grain throughout the solid, and are used in a variety of electronic, optical, and other applications. Single crystals have neither internal grain boundaries nor differing internal crystal orientations that cause variations in properties at different locations within the solid, and are normally low in defect and impurity concentrations. For these reasons single crystals exhibit predictable, uniform, and favorable properties throughout their volumes.
Most solids normally are found in a polycrystalline state having many grains, and special care must be taken to prepare single crystals. One of the common methods of preparing large single crystals is to transform a polycrystal rod of a material to a single crystal by the "float zone" method. In one embodiment of this technique, a polycrystalline, cylindrical rod of the material to be transformed is mounted with its cylindrical axis vertical. A local heater having sufficient power to melt a segment of the rod to form a molten zone having a solid piece at either end is positioned around the rod. The heater is then moved upwardly relative to the rod at a precise rate of travel, so that the molten zone progresses along the length of the rod. When the process is properly accomplished, the molten zone solidifies as a single crystal that extends for a portion of the length of the rod. The polycrystalline end pieces are cut off and discarded, and a cylindrical single crystal rod remains.
The polycrystalline rod used as the starting material of the float zone process must be uniformly dense throughout to avoid irregularities in the final single crystal. The rod should also be nearly fully dense so that a relatively low level of emission of gas, or "outgassing", occurs during solidification, and so that a "waist" of reduced section and low mechanical stability is not formed in the molten zone. The rod must also be physically straight and uniform so as to be mountable in the float zone apparatus. These requirements are easily met when the starting rod is a solid piece produced, for example, by casting molten metal into a cylindrical mold. However, many materials such as refractory carbides and nitrides cannot be readily prepared by casting, metalworking, or a like technique that produces a fully solid rod. Such refractory materials usually have high melting points and are sensitive to impurities that make melting and casting difficult or impossible, and there may be no suitable crucible for preparing a melt.
Another approach to providing the polycrystalline rod starting material is to form the rod from a solid powder by a prior fabrication process, thereby producing a "preform" that is then processed to a single crystal by float zoning. There are many different powder processing approaches, and one is currently used in an attempt to prepare refractory material preforms. The refractory material preform is prepared by mixing the appropriate composition of powder with a binder material such as camphor dissolved in alcohol, uniaxially compacting the powder and binder into a cylindrical mold at ambient temperature, enclosing the pressed powder in a rubber envelope and isostatically pressing at ambient temperature, sintering the rod to form a solid with sufficient strength to be handled, and grinding the sintered rod to a cylindrical shape. Alternatively, the powder can be isostatically hot pressed, but this is a costly process.
The techniques presently used to prepare preforms suffer from several shortcomings. It is difficult, if not impossible, to prepare preforms that are fully or nearly fully dense by cold isostatic pressing and subsequent sintering. For some preform materials, such as titanium carbide, the best densities that can be obtained by these methods are about 80% to 85% of the theoretical maximum density. Consequently, about 15% to 20% of the volume of the preform is voids that can absorb gas. The gas is emitted during float zone processing, and the violent outgassing interferes with the single crystal growth and causes instability in the floating zone. Also, compaction of powders in a die normally results in density variations of the compacted powder because of friction between the powder and the die walls. The density variations cause the preform to become bowed upon sintering, so that it cannot be physically fitted into the float zone apparatus.
Thus, while the float zone technique has been proven as a means of preparing single crystals, in certain circumstances it suffers from limitations resulting from the inability to prepare the required preforms from powder. There therefore exists a need for an improved technique for preparing rods from powdered material for use as preforms in float zone growth of single crystals. This need is acute for refractory materials such as titanium carbide, which are not readily prepared as preforms by conventional melting and casting or other direct technique, and instead must be prepared from powders. The present invention fulfills this need, and further provides related advantages.